The invention relates to an arrangement for the regulation of the electron beam power of an electron gun.
Electron guns are primarily utilized for melting, welding, working and evaporation of all types, preferably in vacuo. It is here often required to adapt the power of the electron beam to varying conditions.
A frequently utilized electron gun is the three-electrode electron gun, which comprises a filament, a block cathode and an anode. As a rule, the filament, also referred to as primary cathode, is comprised of a tungsten wire spiral, through which current flows. In the vacuum, due to thermal emission, electrons are emitted from the surface of the wire spiral in sufficient quantities. In front of the filament is located the block cathode, also referred to as secondary cathode, which is intended to emit the high-power electron beam.
To accomplish this, significantly more electrons must be emitted from the block cathode than is the case with the filament. To achieve this emission efficiency of the block cathode, between the filament and the block cathode an electric voltage, the so-called block voltage, is applied. It is of the order of magnitude of up to 3 kV.
This block voltage generates an electron beam between filament and block cathode, which causes intense heating of the block cathode. This manifests itself optically in a bright luminous effect. The electrons discharged in this manner from the block cathode can absorb kinetic energy in an electric field applied between block cathode and anode.
The electric field is generated as a rule by a voltage of the order of magnitude of 40 kV. The block cathode should herein emit as many electrons as are required for providing the desired beam power.
If the emission of electrons is to be kept constant, the temperature of the block or secondary cathode must also be kept constant. A method is already known (DD 70 674) for keeping the emission temperature of the secondary cathode constant in electron guns heated by electron bombardment for electron beam welding, melting and evaporation devices. In this connection, the emission current of the primary cathode is regulated via the heating current of the primary cathode at constant acceleration voltage between the primary cathode and secondary cathode by means of a controlled regulating variable, derived from the heating current, in which the heating current of the primary cathode is limited via a controlled regulating variable derived therefrom The emission current of the block or secondary cathode, also referred to as main cathode, is herein neither acquired nor regulated.
In a known circuit for the regulation of the operating parameters of an electron beam generator a regulating means for the filament cathode is provided, which is comprised of three individual regulators connected in cascade (DE 23 25 808 C2). These individual regulators are a heating current regulator, an emission current regulator and a beam current regulator.
High-power electron and beam guns are often operated at constant voltage over relatively long time periods and at constant emission current. Due to the high thermal inertia of the block cathodes, rapid variation of the power is not possible through the temperature change of the block cathode. Fast changes of the electron beam power could be effected through a negative bias voltage of a Wehnelt [modulator] cylinder. However, since the emission area of the block cathode is very large, the Wehnelt voltage would need to be very high in order to attain therewith a power control (W. Dietrich, H. Ranke and H. Stumpp: Strahlführung und Leistungsteuerung von Elektronenstrahlkanonen bis 600 kW und deren Einsatz in der industriellen Produktion, Optik 7-2001, p. 11).
A fast power change could also be attained through the variation of the acceleration voltage, if the cathode is operated in the space charge region. Of disadvantage in this method would be the simultaneously required adaption of the focusing to the changed acceleration voltage in order to obtain an electron beam position independent thereof (W. Dietrich et al., loc. cit.).
Furthermore a device for electron guns is known which comprises a regulator with which the heating current and/or the emission current of the filament can be regulated (DE 40 31 286 A1). The regulator has here a filament heating current regulator and an emission current regulator, and to the filament heating current regulator is supplied the difference between the nominal value of the filament heating current and the instantaneous value of the filament heating current, while to the emission current regulator is supplied the difference between the nominal value of the emission current and the instantaneous value of the emission current. The advantage of this device lies therein that the filament heating system can be optionally regulated via the heating current or the emission current.
In addition, a method for the regulation of the electron beam power of electron sources is also known, with which the regulation of the nominal power from zero to 200 kW is possible (DE 102 42 538 A1). The electron source herein comprises an indirectly heated cathode, an anode as well a focusing electrode. At least two independent correcting elements are provided, each of which varies one of the parameters ‘cathode temperature, cathode voltage, focusing electrode voltage and cathode-anode distance’ as control parameters.
Lastly, a device for the automatic regulation of the operating state of an electrothermal installation with an electron gun is known, whose main cathode is heated by an electron bunch emitted by an auxiliary cathode (DE 19 35 710). The main cathode of the electron gun is here supplied from a first structural unit and the auxiliary cathode from a second structural unit. In addition, a circuit section for the correction of the main cathode heating is provided, which is connected with at least one of said structural units. With this known device the stability of the operating state of an electron beam installation can be improved. However, a rapid change of the beam power is not associated therewith.
The aim of the invention is to change rapidly the beam power of an electron gun.
This aim is attained with the arrangement according to the present invention.
The invention consequently relates to an arrangement for the regulation of the electron beam power of an electron gun, which comprises a filament cathode, a block cathode and an anode. Between the filament cathode and the block cathode is applied a first voltage, while between the block cathode and the anode a second voltage is applied. With the aid of a first closed-loop regulated system the filament cathode is regulated to a constant current value, which has a filament temperature, which is sufficient for the maximum beam power of the block cathode. A second closed-loop regulated system which includes a block power regulator, which is acted upon by the difference between the instantaneous value of the block power and nominal value of the block power, regulates the voltage between the filament cathode and the block cathode.
The advantage attained with the invention lies in particular therein that it is not necessary to provide three regulators connected in cascade. The regulation becomes thereby faster and disturbances can be better managed. In addition, the closed-loop gain is decreased whereby small disturbances in the block power have less serious effects on the output. The block power, further, is regulated, which represents the temperature better than the block current.
An embodiment example of the invention is shown in the drawing and will be described in further detail in the following.